The envelope gene of several retroviruses, including HIV, appears to play an important role in pathogenesis. Dr. Orit Nahor tried to construct a non-infectious derivative of HIV that would express envelope constitutively or conditionally in stably transfected cells. Starting with gag-pol deleted HIV clone that expressed envelope in transient transfection assays, Dr. Nahor substituted the hygromycin resistance marker for part of the nef gene. For reasons we do not yet understand, this interfered with envelope expression. Additional constructs are being pursued with the goal of investigating whether envelope expression, per se, is cytopathic, and whether cytopathicity requires co-expression of CD4. Dr. Kazunobu Fujita, working in collaboration with Dr. Keith Peden, investigated molecular changes in HIV associated with adaptation to growth in tissue culture cell liens. Dr. Peden had developed and characterized an infectious molecular clone of HIV that showed adaptation to growth in H9 and U937 cells after forced passage in H9 cells. Dr. Fujita used PCR and a screening technique sensitive to single base changes in bulk amplified DNA, to identify two regions in the envelope gene that differed between the adapted and parental viruses. Dr. Peden showed that these changes were sufficient for enhanced growth in H9 and U937 cells. The location of the changes suggests that alteration in binding of HIV to CD4 and propensity to fuse after binding are responsible for enhanced growth in certain cell lines.